The present invention generally relates to solid oxide fuel cells (SOFCs) and, more particularly, to a multilayered, multifunctional electrolyte in solid oxide fuel cells having high mechanical strength, high ionic conductivity, stability in air and fuel, chemical compatibility to other portions of the fuel cell, and reduced temperature operation.
A solid oxide fuel cell is an energy conversion device that produces direct-current electricity by electrochemically reacting a gaseous fuel (e.g., hydrogen) with an oxidant (e.g., oxygen) across an oxide electrolyte. The key features of current SOFC technology include all solid-state construction, multi-fuel capability, and high-temperature operation. Because of these features, the SOFC has the potential to be a high-performance, clean and efficient power source and has been under development for a variety of power generation applications.
Under typical operating conditions, an SOFC single cell produces less than 1 V. Thus, for practical applications, single cells are stacked in electrical series to build voltage. Stacking is provided by a component, referred to as an interconnect, that electrically connects the anode of one cell to the cathode of the next cell in a stack. Conventional SOFCs are operated at about 1000xc2x0 C. and ambient pressure.
An SOFC single cell is a ceramic tri-layer consisting of an oxide electrolyte sandwiched between an anode and a cathode. The conventional SOFC materials are yttria-stabilized zirconia (YSZ) for the electrolyte, strontium-doped lanthanum manganite (LSM) for the cathode, nickel/YSZ for the anode, and doped lanthanum chromite for the interconnect. Currently, there are two basic cell constructions for SOFCs: electrolyte-supported and electrode-supported.
In an electrolyte-supported cell, the electrolyte is the mechanical support structure of the cell, with a thickness typically between 150 and 250 xcexcm. Electrolyte-supported cells are used, for example, in certain planar SOFC designs. In an electrode-supported cell, one of the electrodes (i.e., the anode or cathode) is the support structure. The electrolyte is a thin film (not greater than 50 xcexcm) that is formed on the support electrode. Tubular, segmented-cells-in-electrical-series, and certain planar SOFC designs, employ this type of cell.
Conventional YSZ-based SOFCs typically employ electrolytes.thicker than 50 xcexcm and require an operating temperature of 1000xc2x0 C. to minimize electrolyte ohmic losses. The high-temperature operation imposes stringent material and processing requirements to the fuel cell system. Thus, the recent trend in the development of SOFCs is to reduce the operating temperature below 800xc2x0 C. The advantages of reduced temperature operation for the SOFC include a wider choice of materials, longer cell life, reduced thermal stress, improved reliability, and potentially reduced fuel cell cost. Another important advantage of reduced temperature operation is the possibility of using low-cost metals for the interconnect.
Various attempts have been made to reduce the operating temperature of YSZ-based SOFCs while maintaining operating efficiency. One attempted method reduces the thickness of the electrolyte to minimize resistance losses. Various methods have been evaluated for making cells with thin films (about 5 to 25 xcexcm thick). Electrode-supported cells (specifically, anode-supported cells) with thin electrolyte films have shown high performance at reduced temperatures. Power densities over 1 W/cm2 at 800xc2x0 C. have been reported, for example, in de Souza et al., YSZ-Thin-Film Electrolyte for Low-Temperature Solid Oxide Fuel Cell, Proc. 2nd Euro. SOFC Forum, 2, 677-685 (1996); de Souza et al., Thin-film solid oxide fuel cell with high performance at low-temperature, Solid State Ionics, 98, 57-61 (1997); Kim et al., Polarization Effects in Intermediate Temperature, Anode-Supported Solid Oxide Fuel Cells, J. Electrochem. Soc., 146 (1), 69-78 (1999); Minh, Development of Thin-Film Solid Oxide Fuel Cells for Power-Generation Applications, Proc. 4th Int""l Symp. On SOFCs, 138-145 (1995); Minh et al., High-performance reduced-temperature SOFC technology, Int""l Newsletter Fuel Cell Bulletin, No. 6, 9-11 (1999). An alternative attempt at reducing operating temperature has involved the use of alternate solid electrolyte materials with ionic conductivity higher than YSZ, as described in Minh, Ceramic Fuel Cells, J. Am. Ceram. Soc., 76 [3], 563-88 (1993). However, the work on alternate electrolyte materials is still at a very early stage.
The electrolyte and cathode have been identified as barriers to achieving efficiency at reduced operating temperatures due to their significant performance losses in current cell materials and configurations. With YSZ electrolyte-supported cells, the conductivity of YSZ requires an operating temperature of about 1000xc2x0 C. for efficient operation. For example, at about 1000xc2x0 C. for a YSZ electrolyte thickness of about 150 xcexcm and about a 1 cm2 area, the resistance of the electrolyte is about 0.15 ohm based on a conductivity of about 0.1 S/cm. The area-specific resistance (ASR) of the electrolyte is, therefore, about 0.15 ohm-cm2. For efficient operation, a high-performance cell with an ASR of about 0.05 ohm-cm2 is desired. To achieve an ASR about 0.05 ohm-cm2 at reduced temperature operation (for example, 800xc2x0 C.), the required thickness (i.e., 15 xcexcm) of YSZ can be calculated. If the desired operating temperature is less than 800xc2x0 C., while the ASR remains the same, either the thickness of YSZ must be further reduced or highly conductive alternate electrolyte materials must be used.
For alternate electrolyte materials in SOFCs, the desired operating temperature determines the choice of materials to achieve high performance. The conductivity and stability of the electrolyte are two key parameters in the selection of the electrolyte material. The highest ionic conductivities are typically found in the fluorite, perovskite, and brownmillerite structures, as indicated by Boivin et al., Chem Mater., 10, p. 2870 (1998). These include doped materials of Bi2O3, CeO2, LaGaO3, and Srxe2x80x94Fexe2x80x94Co oxides. Of these materials, doped Bi2O3, is unstable in a fuel atmosphere and doped Sr2Fe2O5 is a mixed ionic and electronic conductor. Therefore, these two materials are unsuitable for use as SOFC electrolytes. Doped CeO2 in one layer and YBa2Cu3O7 in another layer of a bilayer electrolyte have been attempted to increase the open circuit voltage (OCV) in U.S. Pat. No. 5,731,097. Another bilayer electrolyte is shown in U.S. Pat. No. 5,725,965, wherein one layer of doped Bi2O3 is protected from the fuel environment by a protective layer of doped CeO2.
At reduced operating temperatures of about 550 to 700xc2x0 C., ceria (CeO2) doped with Gd (CGO) and lanthanum gallate (LaGaO3) doped with Sr, Mg (LSGM) or Fe in addition to Sr and Mg (LSGMFe) have been considered due to their high conductivity [Feng et al., Eur. J. Solid St. Inorg. Chem., 31, p. 663 (1994); Huang et al., Superior Perovskite Oxide-Ion Conductor; Strontium- and Magnesium-Doped LaGaO3: I, Phase Relationships and Electrical Properties, J. Am. Ceram. Soc., 81, [10], 2565-75 (1998); Steele, Oxygen transport and exchange in oxide ceramics, J. Power Sources, 49, 1-14 (1994); Ishihara et al., Intermediate Temperature Solid Oxide Fuel Cells Using LaGaO3 Electrolyte Doped with Transition Metal Cations, Proc. Electrochem. Soc. Mtg., Seattle, May 2-5 (1999)]. For example, a 15 xcexcm LSGMFe electrolyte can be operated at about 525xc2x0 C. with an ASR of 0.05 ohm-cm2. Of the materials above, CGO has significant electronic conductivity above 500xc2x0 C. in the fuel atmosphere, leading to low open circuit voltage and decreased fuel cell efficiency.
Therefore, to be a useful electrolyte, CGO must either be used at 500xc2x0 C. or lower according to Doshi et al., Development of Solid-Oxide Fuel Cells That Operate at 500xc2x0 C., J. Electrochem. Soc., 146 (4), 1273-1278 (1999), or modified or protected against reduction by the fuel environment. LSGM (La0.9Sr0.1Ga0.8Mg0.2O3) and LSGMFe (La0.9Sr0.1Ga0.8Mg0.17Fe0.3O3) have high ionic conductivity but react with the Ni typically used in anodes. In addition, some loss of the gallium element occurs from evaporation in the fuel atmosphere over time. Accordingly, cell performance degrades over time. Therefore, a gallate-based electrolyte needs to be protected against interactions with the nickel of the anode and elemental losses. An ideal electrolyte would have the high ionic conductivity of LSGMFe, the chemical compatibility of YSZ or CeO2 with Ni, and the mechanical strength of YSZ.
The fabrication process that the above materials undergo is an important factor that affects the performance of a fuel cell. Several techniques are available to manufacture cells in either of the two classes of cell construction (i.e., electrolyte-supported and electrode-supported), including thick-film electrolytes and thin-film electrolytes.
The term xe2x80x9cthick-film electrolytexe2x80x9d is used to describe self-supported electrolytes used as a substrate to which electrodes are added. Self-supported electrolytes require sufficient thickness (i.e., 150 to 250 xcexcm) for practical handling. Tape casting is typically used to fabricate these dense membranes. During tape casting, a slurry of fine ceramic particles dispersed in a fluid vehicle is cast as a thin tape on a carrier substrate using a doctor blade. The tape is then dried, removed from the carrier substrate, and fired to produce a dense substrate. After sintering, deposition techniques such as hand painting, screen-printing, or spray coating are used to attach electrodes to both sides. The high ohmic resistance of the thick electrolyte necessitates higher operating temperatures of around 1000xc2x0 C. to reduce the ohmic polarization losses due to the electrolyte.
Driven by the benefits of reducing ohmic loss in the electrolyte at lower temperatures (i.e., 550 to 800xc2x0 C.), SOFC development efforts have focused attention on xe2x80x9cthin-film electrolytesxe2x80x9d (i.e., 5 to 25 xcexcm) supported on thick electrodes, such as described in U.S. Pat. No. 5,741,406. A number of selected fabrication processes used for making SOFCs, especially thin YSZ electrolytes, is listed in Table 1.
Other thin-film techniques investigated for SOFC applications include vapor-phase electrolytic deposition, vacuum evaporation, liquid-injection plasma spraying, laser spraying, jet vapor deposition, transfer printing, coat mix process, sedimentation method, electrostatic spray pyrolysis, and plasma metal organic chemical vapor deposition.
Additional related references are found in U.S. Pat. Nos. 5,922,486; 5,712,055; and 5,306,411.
As can be seen, there is a need for an SOFC fabrication process that ensures that no condition or environment in any process step destroys the desired characteristics of any of the materials. Low cost and scalability in the fabrication process is also needed. An electrolyte for use in an SOFC is needed that provides high ionic conductivity, chemical compatibility with Ni or other transition metals, and mechanical strength. In particular, an SOFC electrolyte is needed that can be used at reduced operating temperatures not greater than about 800xc2x0 C., while still providing high performance, including an ASR about 0.05 ohm-cm2. A thin-film electrolyte about 5 to 25 xcexcm thick is also needed for use at reduced operating temperatures. An electrolyte with high strength that is resistant to a fuel environment is another need.
The integrated approach of the present invention provides a high-performance, reduced-temperature SOFC. The approach is based on materials and structures which, when combined, are capable of increased performance in about the 550 to 800xc2x0 C. operating range while maintaining functional integrity up to about 1000xc2x0 C. The materials and fabrication process are economical, scalable, and amenable to high-volume manufacture of fuel cells.
The present invention is based on a thin-film, multilayer, multifunction electrolyte supported on an anode substrate to minimize ohmic losses. Although new electrolyte materials with high conductivities have been reported, each one has inherent problems that need to be overcome, e.g., chemical compatibility, electronic conductivity, and mechanical strength. The present invention is a multilayer composite of highly conductive electrolytes that provides a combination of superior properties while avoiding the problems mentioned above.
In one aspect of the present invention, a solid oxide fuel cell comprises an anode; a cathode opposite to the anode; and an electrolyte between the anode and cathode, with the electrolyte including a barrier layer proximate to the anode, and the barrier layer preventing chemical interactions between the electrolyte and anode in addition to preventing elemental losses from the electrolyte.
In a further aspect of the present invention, a solid oxide fuel cell comprises an anode; a cathode opposite to the anode; and an electrolyte between the anode and cathode, with the electrolyte including a strengthening layer proximate to the cathode, and the strengthening layer having alternating layer elements that provide fracture resistance to the electrolyte.
In another aspect of the present invention, a method of making a solid oxide fuel cell comprises forming an electrolyte between an anode and a cathode; minimizing elemental losses from the electrolyte by the use of a barrier layer proximate to the anode; and preventing chemical interactions between the electrolyte and anode by the use of the barrier layer.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.